As heat-resistant components for turbine rotor blades and turbine stator blades for jet engines and industrial gas turbines, Ni-based superalloys with improved heatproof temperature have been developed. With these heat-resistant components for turbine rotor blades and turbine stator blades, to further improve durability, a ceramic thermal barrier coating is widely used. Furthermore, to promote adhesion of the ceramic thermal barrier coating to Ni-based superalloy jet engine parts and prolong parts life, a number of highly oxidation-resistant bond coat materials have been proposed for use between a Ni-based superalloy component and a ceramic thermal barrier coating material. These bond coating materials are, in most cases, Al-containing alloys such as Ni- or Co-aluminide, MCrAlY (M is at least one metal selected from Ni, Co and Fe), and platinum-aluminide (Patent Literatures 1 to 4).
However, if a turbine blade made of a Ni-based superalloy adopting these bond coat materials is used at a high temperature for a long time, elemental interdiffusion progresses in proximity to the interface between the Ni-base superalloy substrate and the bond coat material and/or between the ceramic thermal barrier coat material and the bond coat material. Consequently, material technological problems, such as material degradation and decrease in durability of the Ni-based superalloy and decrease in durability of the bond coat material, arise, resulting in decrease in durability of turbine blades themselves. In particular, since gas temperature of jet engines and gas turbines has recently been increasing, turbine blade temperature is also increasing, thereby further accelerating the occurrence of a phenomenon of elemental interdiffusion. In addition, high-pressure turbine blades have a hollow structure to facilitate cooling, and because thinning is progressing, suppression of elemental interdiffusion is becoming an increasingly significant technical challenge.
To suppress elemental interdiffusion in the proximity of the interface between a Ni-base superalloy substrate and a bond coat material, a diffusion barrier coating has been studied, but sufficient effect has yet to be obtained.
Paying attention to the fact that elemental interdiffusion occurs, thereby causing Ni-base superalloy substrate to degrade, because the alloy composition of the Ni-base superalloy substrate and that of the bond coat material are not in a state of thermodynamic equilibrium, the inventors have proposed to use an alloy that is in a state of thermodynamic equilibrium with a Ni-base superalloy substrate as a bond coat material(hereinafter referred to as EQ coat material). Specifically, it has been clarified that by coating one or more layers containing at least one of γ phase, γ′ phase, and B2 phase having a composition that is in a state of thermodynamic equilibrium with the Ni-base superalloy substrate on the Ni-base superalloy substrate, elemental diffusion is suppressed significantly, and that degradation of the coated Ni-based superalloy can thus be suppressed (Patent Literature 5).
In the case heat-resistant gas turbine components using a Ni-based superalloy as a substrate, in order to increase heat resistance of the Ni-base superalloy substrate, currently, a ceramic thermal barrier coat layer is coated on the surface of the substrate. However, since long-term adhesion property of the interface between the Ni-base superalloy substrate and the ceramic thermal barrier coat layer is insufficient, the Ni-base superalloy substrate and the ceramic thermal barrier coat layer are bonded via a bond coat layer. The bond coat layer is made of a bond coat material. Of various bond coat materials that have thus far been proposed, EQ coat material is an excellent bond coat material capable of suppressing generation of a secondary reaction zone (SRZ), which occurs in proximity to the interface between the Ni-base superalloy substrate and the bond coat layer. However, even if the EQ coat material is used, detachment of ceramic coat layer occurs as a result of generation of an oxide layer under high-temperature oxidizing conditions in proximity to the interface between the ceramic thermal barrier coat layer and the EQ coat material. Thus, the life of Ni-based superalloy components is not necessarily considered to be sufficient.